Polythiophene polymers, and especially poly-(3,4-ethylenedioxythiophene) (PEDT) have been found to be a very favorable electrically conductive material for many systems wherein high conductivity in a coated layer is desired. Polythiophene polymers can be made by in-situ polymerization or formed separately and then used as a polymer slurry to coat a surface. The formation of the polymer from a monomer is known to include some fraction of dimer and oligomer formation wherein the dimer or oligomer is incorporated into the polymer chain. Incorporation of particularly, dimers has been found to be advantageous to the polymerization process. It has been known for some time that the dimer or oligomer formed under common conditions of in situ polymerization is largely unconjugated and these unconjugated segments disrupt the conjugation within the polymer. There has therefore been a desire to eliminate, or at least significantly reduce, the formation of unconjugated dimers and oligomers even though this has proven quite difficult.
Purposefully and preferentially synthesizing the conjugated dimer and oligomer in high ratios requires very harsh chemicals and conditions, and is typically done by Ullmann coupling. Ullmann coupling is not a desirable reaction for use on a large manufacturing scale. Converting the non-conjugated dimer to the conjugated dimer has been described. However, this process requires a dehydrogenating agent and is not a practical process for a manufacturing environment, either.
Polythiophenes have gained a significant following in the art of electrolytic capacitors and polythiophenes are now commonly employed as the charge collecting layer, or cathode, in solid electrolyte capacitors. The incorporation of non-conjugated dimers and oligomers, and resulting loss of conductivity, is particularly bothersome for capacitors and the processes associated with purposeful synthesis of conjugated dimers or conversion of non-conjugated dimers to conjugated dimers has not been compatible with capacitors and capacitor manufacturing processes.
Both chemical and electrochemical polymerization has been used to form intrinsically conductive polymers for electrolytic capacitors. Chemical polymerization is well described in U.S. Pat. No. 4,910,645, to Jonas et. al., U.S. Pat. No. 6,136,176 to D. Wheeler, et. al. and U.S. Pat. No. 6,334,966 to Hahn et al. The process consists of immersing the anodized substrate first in a solution of an oxidizing agent such as, but not necessarily limited to, Fe (III) p-tosylate. After a drying step the anode bodies are then immersed in a solution of the monomer. Once the solution of monomer, which may consist entirely of monomer, is introduced into the capacitor anode bodies, the anodes are allowed to stand to facilitate production of the intrinsically conductive polymer material. Repeated dipping sequences may be employed to more completely fill the pore structure of the anode bodies and to cover the surface of the anode. In practice, rinsing cycles are generally employed to remove reaction by-products, such as ammonium sulfate, sulfuric acid, or iron salts when an iron (III) oxidizer is employed, or other by-products depending on the system employed.
When used in a capacitor a decrease in conjugation length of the polymer deteriorates the conductivity which causes an increase in equivalent series resistance (ESR) and leakage current. The acid present in the oxidizer, and as a by-product of the polymerization, promotes the formation of non-conjugated linkages in the polymer. In U.S. Pat. No. 7,754,276 procedures to control the acid content in the monomer solution are disclosed. Although the conductivity of polymer made according to U.S. Pat. No. 7,754,276 remained high, the growth rate of the conductive polymer could be decreased. More production cycles may be required to provide adequate polymer coverage.
Chemical production of intrinsically conductive organic polymers may also be carried-out on capacitor anode bodies by first introducing the monomer to the capacitor bodies, followed by introduction of the oxidizer and dopant which is the reverse order of polymer precursor introduction described above. It is also known to mix the dopant acid(s) with the monomer solution rather than with the oxidizer solution if this is found to be advantageous. U.S. Pat. No. 6,001,281 and U.S. Pat. No. 6,056,899 describe a chemical means of producing an intrinsically conductive organic polymer through the use of a single solution which contains both the monomer and the oxidizing agent, which has been rendered temporarily inactive via complexing with a high vapor pressure solvent. As the solution is warmed and the inhibiting solvent evaporates, the oxidative production of conductive polymers ensues. The dopant acid anion is also contained in the stabilized poly-precursor solution.
Intrinsically conductive organic polymers generally contain one dopant anion for each 3 to 4 monomer units which have been joined to form the polymer. The presence of a strong dopant acid anion is thought to result in a delocalization of electric charge on the conjugated molecular chain and therefore provides electrical conductivity. In the case of a ferric salt being used as the oxidizer, the presence of an acid also keeps the Fe3+ ions from precipitating out of the solution. In the sequential dipping process the acid could accumulate in the monomer solution. It is known that an acid can promote the formation of non-conjugated dimers and trimers through acid catalyzed reaction. U.S. Pat. No. 6,891,016 to Reuter et al. disclosed the formation of non-conjugated ethylenedioxythiophene (EDT) dimer, and trimer in the presence of an acid catalyst.
There has been a long standing need for a process for forming a conductive polymer, and particularly a polythiophene, with a low number of unconjugated sections. Such a process is provided herein.